1. Field of the Invention
The present invention relates to materials and devices for optical communications, and more particularly to electro-optic ceramic materials having high transparency and electro-optic activity and devices constructed using such materials.
2. Technical Background
While much progress has been made in the last thirty years in developing optical switches or modulators, current devices are not satisfactory for every application. Many active optical devices, such as intensity attenuators, used in present-day systems are based on electromechanical actuation. In one type of conventional device, optical fibers are positioned end to end and mechanically moved into or out of line. In another type of conventional device, mirrors are rotated to direct beams into or away from a receiving fiber. This can be accomplished mechanically, or with piezoelectric or electrostatic devices. Devices based on motion of components have slow switching times, and may have unacceptable environmental or long-term stability.
Optical devices without moving parts have been designed to address some of the switching speed and stability problems mentioned above. These devices depend on materials that can change optical properties without bulk motion, such as liquid crystalline materials and electro-optic crystal materials. Liquid crystals tend to have relatively slow switching speeds, as the mechanism of actuation is rotation of entire molecules under the influence of an electric field. Electro-optic crystal materials such as LiNbO3 have much higher switching speeds, but are extremely polarization dependent.
One especially promising class of materials for use in active optical devices is electro-optic ceramic materials. Electro-optic ceramics are generally composed of ferroelectric complex oxides, and are polycrystalline. Due to the random orientations of the individual crystalline grains, electro-optic ceramics are optically isotropic in the absence of an electric field. In the presence of an electric field, electro-optic ceramic materials become anisotropic, with a lower refractive index in the direction of the field than perpendicular to the field. The material may be switched between isotropic and anisotropic states by controlling the electric field. For use in electro-optic applications, it is desirable that electro-optic ceramic materials have high transparency, high quadratic electro-optic coefficients and low switching hysteresis.
Lead lanthanum zirconate titanate (PLZT) is the most common electro-optic ceramic material. PLZT materials can be formed to be substantially transparent to light having wavelengths in the range of 0.5 μm to 7 μm. Properties of PLZT compositions can be tuned by adjusting the relative amounts of lead, lanthanum, zirconium and titanium. For example, a PLZT composition having the formula Pb0.91La0.09(Zr0.65Ti0.35)0.9775O3 has a quadratic electro-optic coefficient R=9.2×10−16 m2/V2, but has extremely large hysteresis at low fields, making it unsuitable for high-speed electro-optic applications. By increasing the lanthanum concentration, one can improve hysteresis at the expense of electro-optic activity. For example, a PLZT composition having the formula Pb0.9025La0.0975(Zr0.65Ti0.35)0.975625O3 has a lower quadratic electro-optic coefficient (R˜2.5×10−16 m2/V2), but has temperature performance and hysteresis suitable for use in devices for optical telecommunications. PLZT materials also suffer from high brittleness and low toughness. Attempts to find new electro-optic ceramics have generally failed to provide an electro-optic ceramic material that has high transparency, a high quadratic electro-optic coefficient, and little hysteresis.